MCUs Simplify Resolver-Based Motor Control Systems

Resolvers, or rotary-position sensors, provide a continuous output of sine and cosine signals that let control electronics determine a shaft's position. Resolvers attached to motors provide information about rotary speed and position for material cutters, printing presses, and similar equipment.

The resolver's sine and cosine signals require processing to convert them into digital values for calculation of position, revolutions per second, and so on. You can buy resolver-to-digital converters, but a fast microcontroller can do the job just as well. And the MCU can create the sine wave signal used to excite the resolver's input.

ICs in the new Delfino TMS320C2000 MCU family from Texas Instruments -- with several external components for signal conditioning -- can serve as precision synchro-to-digital converters. The MCUs also supply the hardware necessary to control motors. This application whitepaper by Ramesh Ramamoorthy, a C2000 MCU applications engineer, reviews the principles of resolver operations, describes needed signal conditioning steps, and provides a schematic diagram for external signal conditioning circuits. Those circuits need five op-amps and about 40 resistors and capacitors. TI said in a press release that programmers and engineers can get the free Delfino MCU software for the resolver applications from their local TI sales representative.

According to TI, a Delfino TMS320F28335 MCU will sample the resolver sine and cosine signals at 160 ksamples/sec, which means 16 samples per sine or cosine wave. After an initial offset compensation, the digital values pass to a control block, from which software provides angular values. The MCU also creates the excitation sine wave signal and updates it at the same sample rate. To enhance measurement accuracy, ADC sampling occurs at the peak of the sine and cosine signals received from the resolver.

Those interested in exploring the Delfino MCU capabilities and software tools can start with the free C2000 controlSUITE and Code Composer Studio integrated development environment (though there's a code limit on the free version).

An 'F28335 controlCARD (TMDSCNCD28335) costs $69 and gives you an MCU and other circuits on a 9cm x 2.5cm board that provides a standard 100-pin DIMM-type edge connector. The DM100 F28335 Experimenter's Kit (TMDSDOCK28335) costs $99 and supplies a baseboard with built-in USB-JTAG-emulation hardware (shown in the photo). TI also sells a variety of C2000 motor control kits that are compatible with Delfino microcontrollers.

Yes, semiconductors do go away. A company called Reticon--acquired years ago by EG&G--made a programmable filter I liked very much. No replacement. Likewise, Intersil manufactured a dual-slope (integrating) amalog-to-digital converter. That went away, too, although Maxim made the chip, too, but I haven't checked recently. There are some secondary-market companies that have stocks of older ICs for companies that need replacements.

There's a balance between choosing recent semiconductors that have some history and longevity vs. picking the latest-and-greatest devices that might be a "flash in the pan." Engineers and product designers should always ask a supplier about the longevity of a device or product line.

Yes, it is true that the 8051 is still around, and in quite a few variants, as well. But try to find an 8047. And of course the *86 is still around. But neither of those would be considered a "small" processor, I don't think. Also, it looks a bit like many of the newer crop of small devices are constantly changing, hoping to find a configuration that is what everybody wants. The less popular versions don't lkast as long. Sort of like the flat-pack CMOS ICs of a few years back.

Of course there are also those devices that go away because of yield problems, although that may be less common than a few years back. Presently it seems that Analog Devices has stopped producing one of the audio compressor devices, and a few other audio chips, for some reason. So products do go away.

Hi, William. Yes, watch you don't get cut on the "bleeding edge" of new technologies. That said, the venerable 8051 continues to live as does the Z80 family. I believe Zilog still has variants of the Z80 family and Rabbit Semiconductor (now part of Digi International) has modules based on the Z80 architecture. Of course the x86 architecture continues to roll on, too.

Long after whatever MCU that you choose has gone out of production and is not available anywhere, thye same analog ICs will still be available from multiple makers and distributors stock. So if the anticipated product life is measured in days or months, then choose the MCU approach. But if it is a product with an expected lifetime of years, then make it out of sustainable parts. (A new expression?) Cutting edge stuff often causes bleeding.

Hi, AndyT. The "memory" connector is actually the connector for a TI MCU "ControlCard," already in place. The odd perspective of the image places the upper edge of the ControlCard along the same line as the far edge of the small motherboard. Look again and you'll see a board plugged in. The MCU has a lot of memory. The connector lets engineers and programmers use different types of ControlCards.

I have to disagree, 8 bits are still very much alive, especially when combined with a good compiler and plenty of memory, which is now pretty cheap. I was looking into crunching some color graphics a while ago and did two designs; one with an arm and the other with a SiLabs 8051. The 8051 was faster and cheaper in this application.
In most of the small appliances and sensors I work on, an 8 or small 16 bit machine is still my first choice. It's hard to justify a full 32 bit core when I only need 4k of code.

Hi, Island_Al. Yes, plenty of life left in 8-bit MCUs for a wide variety of uses. A few days ago I sketched out a neat circuit for model-railroad enthusiasts that would use an 8-bit PIC in an 8-pin package--and an assembly-language program.

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